The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, thermal efficiency of combustion engines increases as the compression ratio increases, and if ignition timing is advanced to some degree, thermal efficiency of spark-ignition engines increases.
However, if the ignition timing of the spark-ignition engines is advanced at a high compression ratio, abnormal combustion may occur and the engine may be damaged. Thus, the ignition timing cannot be excessively advanced and accordingly engine output may deteriorate.
A variable compression ratio (VCR) apparatus changes the compression ratio of an air-fuel mixture according to a driving condition of the engine.
The variable compression ratio apparatus raises the compression ratio of the air-fuel mixture at a low-load condition of the engine in order to improve fuel mileage. On the contrary, the variable compression ratio apparatus lowers the compression ratio of the air-fuel mixture at a high-load condition of the engine in order to inhibit or prevent occurrence of knocking and improve engine output.
Currently, for diesel engines, in order to meet the tight control of exhaust gas, low temperature combustion is implemented by decreasing a compression ratio by increasing the volume of a piston combustion chamber. Since cold engine start performance is deteriorated according to the decrease of compression ratio, the stiffness of a glow system is strengthened by making it with ceramic material and a separate system for controlling the glow system is added. Consequently, a manufacture cost is increased.
The description in the Background is written to increase understanding of the background of the present disclosure, and it may include descriptions which are not the conventional art that is already known to a person of ordinary skill in the art.